Television system



A. W. VANCE TELEVISION SYSTEM Filed April 2, 1952 INVENTOR Arth u. TWVomce HIS ATTORNEY Patented Dec. 14, 1937 UNITEDl STATES PATENT FFICE dio Corporation of America,

Delaware a corporation of Application April 2, 1932, Serial No. 602,651

16 Claims.

My invention relates to improvements in television systems and, more particularly, to the method of and apparatus for scanning.

Heretofore, development work on television systems, which embody a cathode ray tube at the receiving station, has been directed particularly to systems of the general type disclosed in my copending application filed June 17, 1931, bearing Serial No. 544,959, and assigned to the RCA Victor Company. In these systems, the cathode ray tube is provided at one end with an electron gun comprising at least three electrodes, one of which is a grid or control element. In operation, a ray of electrons is projected from the gun onto a fluorescent screen at the other end of thetube,

the ray being focused on this screen' and deflected to completely scan the same about twenty times a second.

The received picture signals are applied to the grid and are eiective to vary the ray intensity in exact accordance with variations in conditions of light and shade at elemental areas of the object being televised. This method of operation necessitates very high anode potentials in order to develop sufcient light on the uorescent screen, for the reason that the total light over the screen for any picture frame is only the average of the light intensities of the respective spots, varying over a wide range from dim to bright. These high anode potentials work against easy deflection of the ray, and must' be taken into account in the design of the ray-defiecting apparatus.,

Furthermore, in the systems referred. to, the electrodes. comprising the electron gun are provided with very small, central apertures which must be in exact alignment to cause the electrons in the ray to strike the iiuorescent screen over a very small elemental area. If these apertures are slightly out of line, the spot on the screen is not suciently well dened to contribute toward good picture detail.

Another problem inherent in the systems referred to resides in the fact that as the controlgrid bias is modulated in accordance with the received picture signals, there is 'an accompanying variation of position or shape of the spot.

Also, in the prior systemsl referred to, synchronization is a-separate and diicult problem.

With the foregoing in mind, it is one of the objects of my invention to provide an improved television system of the character referred to which avoids the various mentioned dimculties encountered in the systems proposed heretofore.

More speciiically, an object of my invention (Cl. P18-6) is to provide an improved television system of the character referred to wherein there is no requirement for a grid or control element in the cathode ray tube at either the transmitting or receiving station; wherein the method or manner of operation is such that during each pictureframe period the spot on the fluorescent screen at the receiving station is always at substantially maximum intensity whereby it is possible to get sufficient light while employing anode poten- 10 tials substantially lower than those required heretofore, which condition, in turn, contributes toward substantially easier deiiection of the cathode ray; and wherein synchronization does not present itself as a separate problem, but is 15 automatically and easily effected, without the use of special synchronizing apparatus, on account of its being an inherent characteristic of myimproved system.

Other objects and advantages will hereinafter appear. V

In accordance with my invention, cathode ray tubes'are employed at both the transmitting and receiving stations, the rays at all times being at a substantially constant intensity for maxi- 25 mum effectiveness. At the transmitting station, scanning is accomplished by deflecting the ray, during each picture-frame period, at different velocities in both the horizontal and vertical directions, the velocity of ray-deflection in each 30 direction varying inversely proportionately with variation in light intensity from the respective elemental areas of the object being scanned. In this operation, an electrical wave form is developed, and is applied to the ray-deecting means of the receiver tube, whereby the ray is deflected to scan the usual uorescent screen at the varying velocities in both the horizontal and vertical directions. Accordingly, in operation at the receiving station, the light impression from any elemental area of the screen to the eye of the observer is a function of the velocity of scanning movement of the ray over this particular area, the velocity increasing as darker areas of the object are scanned at the transmitting station, and decreasing as lighter areas of the object are scanned. In other words, in accordance with my invention, there is velocity modulation of the scanning movements of the rays at both the transmitting and receiving stations, both rays operating at all times at a substantially constant and relatively high intensity.

My invention resides in the system and method of operation of the character hereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawing, wherein Figure 1 is a diagrammatic view of a television system constructed and operating in accordance with my invention; and

Fig. 2 is a graphical showing, illustrative of the principle of operation in Fig. 1.`

With reference to Fig. 1, the reference numerals III and I2 designate cathode ray tubes of the general type disclosed in my copending application referred to, and provided respectively with electron guns I4 and I6 and fluorescent screens I8 and 20.

The guns I4 and I6 comprise the usual cathodes I3 and I5 and the usual rst anodes I1 and I9, and operate to develop the electron rays or beams 22 and 24, respectively, each at a relatively high and constant intensity for maximum eifectiveness With respect to the fluorescent screens.

The rays 22 and 24 are deflected horizontally by applying a saw-tooth voltage wave, at a suitable line-scanning frequency, to the plates 26 and 28, and are deflected vertically by applying a saw-tooth voltage wave, at a suitable framing frequency, to the plates 30 and 32. Since the same saw-tooth voltage waves are applied to the deecting plates of the two tubes, the rays 22 and 24 scan the screens along identical paths 34 and 36, as indicated, and the position and velocity of scanning movement of the two rays at any instant is the same.

Two relaxation oscillator circuits operate to generate the two voltage waves for horizontal and vertical deiiection of the beams 22 and 24. The rst circuit comprises a condenser 38 charged from a source of constant potential 40. A discharge device such as a glow tube 42 is connected across the condenser, and breaks down each time the condenser voltage reaches the level indicated by the line 44 in Fig. 2. The glow tube 42 maintains a constant breakdown voltage, thereby insuring that the peak values of the oscillations are always at the level 44. A saw-tooth voltage wave is thereby generated, and appears across a resistance 46, from whence it is applied to the horizontal deiiecting plates 26 and 28 through suitable amplifiers 48 and 56, respectively. The charging period for each saw-tooth wave varies, as will hereinafter more fully appear, with variation of the bias on an electron tube 52 connected, as shown, across the condenser 38. I'he capacity of the condenser 38 is adjusted to make the normal speed or frequency of the voltage wave equal to the desired, average line-scanning frequency.

The second relaxation oscillator circuit, the parts of which are designated by the same reference numerals, with the suiiix a, as the corresponding parts in the circuit for horizontal deflection, operates to generate a saw-tooth voltage Wave which appears across the resistance 46a and is applied to the vertical defiecting plates 30 and 32 through suitable amplifiers 48a and 50a, respectively.

The capacity of the condenser 38a is adjusted to make the normal frequency of the voltage wave equal to the desired, average framing frequency. As in the other circuit, the charging period for each saw-tooth wave varies with variation of the bias on the tube 52a.

At the transmitting station, the bright fiuorese cent spot on the screen I 8 is projected onto the object 56 and scans the same according to the method invented by V. K. Zworykin.' The light reiiected from or transmitted through the object, which may be either stationary or moving, excites a photoelectric cell 58 to generate an electrical wave the shape of which is a function of condition of light and shade at the respective elemental areas of the object. 'I'his wave is ampliied by a suitable amplifier 60 having a substantially fiat characteristic over the required frequency range.

A connection 62 extends between the output of the amplifier 60, having the proper number of stages, to the grids of the tubes 52 and 52a which, at any instant, control the charging rates of the condensers 38 and 38a, respectively. The number of amplifying stages is such that as the light intensity from the scanned elemental areas of the object varies in the sense from shade to light, the respective biasing potentials on the grids of the tubes 52 and 52a become proportionately more negative, causing the charging rates of the condensers 38 and 38a to decrease proportionately, or, in other words, causing the rates of horizontal and vertical deflections of the beam 22 to decrease proportionately.

Similarly, as the light intensity from the scanned elemental areas of the object varies in the sense from light to shade,'the respective biasing potentials on the grids of the tubes 52 and 52a become proportionately more positive, causing the charging rates of the condensers 38 and 38a to increase proportionately, or, in other words, causing the rates of horizontal and vertical deflections of the beam 22 to increase proportionately.

For convenience, the output stage of the amplifier 6D is referred to as being of negative polarity.

In further explanation of the operating action, reference is made to Fig. 2, illustrating the wave forms across the resistance 46 for three diiierent degrees of light intensity from the object 56. For example, when a dark portion or elemental area of the object is being scanned, the rate of charge of the condenser 38 will be relatively high, as indicated by the slope of the charging curve 64. At the intersection of this curve with the line 44, the glow tube breaks down to reduce the condenser voltage to zero practically instantaneously. 'I'his cycle is repeated to generate a sawtooth voltage wave 66l wherein the scanning Velocity is appreciably higher than the average scanning velocity which is determined by the "swing" on the grid of the charging tube 52.

When a relatively light portion or elemental area of the object is being scanned, the rate of charge of the condenser 38 will be lower, as indicated by the slope of the charging curve 68, and the saw-tooth voltage wave 10 will be generated.

When a very bright portion or elemental area of the object is being scanned, the rate of charge of the condenser 38 will be relatively low, as indicated by the slope of the charging curve 12, and the saw-tooth voltage wave 14 will be generated.

In like manner, intermediate wave forms will be generated wherein the respective scanning velocities are proportional to the light intensity reiiected from the object at the particular-instants of time.

The wave forms across the resistance 46a can be analyzed in like manner, it being understood that the ratio of variation in the vertical deilection of the beam 22 to the average framing speed will be controlled by the swing on the grid of the charging tube 52a.

At the receiving station, the beam 24 is deflected in absolute synchronism with deiiection of the beam 22. This is accomplished by amplifying and applying the generated voltage wave across the resistance 66 to the horizontal deflecting plates 26 and 23 of the two tubes, as shown, and by amplifying and applying the generated voltage wave across the resistance 66a to the vertical deiiecting plates 30 and 32 of the two tubes. At the receiving station, therefore, when a bright area of the object is being scanned, the beam 24 will move more slowly in both horizontal and vertical directions over the corresponding spot on the uorescent screen Ztl, thereby giving to the eye a relatively high impression of light intensity. In like manner, when a dark area of the object is being scanned, the beam 2t will move more rapidly in both horizontal and vertical directions over the corresponding spot on the screen 20, thereby giving to the eye a relatively low impression of light intensity. For intermediate conditions, the impressions to the eye will be proportional, so that an image of the object will be faithfully reproduced.

From the foregoing, it will be seen that I have provided an improved television system embodying a cathode ray tube at both the transmitting and receiving stations, wherein no grid or control element is necessary in either of these tubes, wherein a good spot of maximum intensity is obtained at every instant in the operating cycle at. substantially lower anode potentials than have been feasible heretofore, wherein the electron beams are more easily deflected, wherein there is no detrimental variation of position or shape of the spot, wherein there is substantially mo-re overall brilliancy of the reproduced picture than has been obtained heretofore, and wherein synchronization'is an inherent characteristic of the system and does not present itself as a separate and difficult problem, as heretofore.

Another important and favorable characteristic of my improved system resides in the fact that the most rapid variations in current with respect to time occur in the scanning of the darker portions of the object. Therefore', if the system is to make use of a limited frequency channel, any loss of the highest frequencies will be manifested by some degree of loss of detail over the dark portions of the picture rather than over the brighter portions thereof. The best detail is therefore over the brighter portions of the picture which generally have the attention of the observers.

While but one embodiment of my invention has been disclosed, wherein metallic conductors extend between the transmitting and receiving stations, it will be understood that' a radio channel or channels may, alternatively, be utilized and that various other modifications within the conception of those skilled in the art are' feasible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In the art of television, the method of operation which comprises developing a stream of electrons and directing the same at screen structure, deflecting said stream to cause the same to scan said structure, and varying the average speed of deflection in accordance with variations in conditions of light and shade from elemental areas of the object being televised.

' 2. In the art of television, the method of operation which comprises developing a stream of electrons and directing the same at screen structure, deflecting said stream to cause the same to scan said structure, and increasing the average speed of deflection upon variation in light intensity from elemental areas of the object being televised in the sense from light to shade, and decreasing the average lspeed7cf deflection upon variation in light intensity from such areas in the sense from shade to light.

3. In the art of television, the method of operation which comprises developing a stream of electrons and directing the same at screen structure, continuously charging and intermittently discharging a condenser to effect deflection of the stream for scanning purposes, and varying the condenser-charging rate in accordance with variations in conditions of light and shade from elemental areas of the object being televised.

4. In the art of television, the method of operation which comprises developing a stream of electrons and directing the same at screening structure, continuously charging and intermittently discharging a condenser to effect deflection of the stream for scanning purposes, increasing the condenser-charging rate upon variation in light intensity from elemental areas of the object beingv televised in the sense from light to shade, and decreasing the condenser-charging rate upon variation in light intensity from such 3 areas in the sense from shade to light.

5. In a television system, scanning means comprising an electron device for developing a stream of electrons, means for deecting saidn stream, and means for effecting variation of the average speed of deflection in accordance with variations in conditions of light and shade from elemental -areas of the object being televised.

, to light.

7. In a television system, scanning means comprising an electron device for developing a stream of electrons, means for deflecting said stream comprising a condenser, means for continuously charging and intermittently discharging said condenser, and means for varying the condensercharging rate in accordance with variations in conditions of light and shade from elemental areas of the object being televised.

8. In a television system, scanning means comprising an electron device for developing a stream of electrons, means for deflecting said stream comprising a' condenser, means for continuously charging and intermittently discharging said condenser, and means for increasing the condenser-charging rate in accordance with variations in light intensity from elemental areas of the object being televised in the sense from light to shade and decreasing such rate in accordance with variations in light intensity from such areas in the sense from shade to light.

9. In a television system, scanning means comprising an electron device for developing a streamof electrons, means for deecting said stream comprising a condenser, means for continuously charging and intermittently discharging said condenser, an electron tube for controlling the con- Izo denser-chargingrate, means for applying a biasing potential to said tube, and means for making said potential more positive upon variation in light intensity from elemental areas of the object being televised in the sense from light to shade and making said potential rmore negative upon variation in light intensity from such areas in the sense from shade to light.

10. In a television system, rscanning means ycomprising an electron device for developing a stream of electrons, means for deecting said stream', said means including a relaxation oscillator circuit, and means for varying the rate of operating action of said circuit in response to and in accordance with variations in conditions of light and shade from elemental areas of the object being televised.

11. In a television system, means for reproducing an image of the televised object, said means comprising a cathode ray tube, means for deilecting the ray, and means for controlling the average rate of deection of the ray in accordance with conditions of light and shade from elemental areas of the object. f 12. In a television system, means for reproducing an image of the televised object, said means comprising a cathode ray tube characterized by the fact that during operation the ray-intensity is substantially constant, means for deiecting the ray horizontally and vertically, and means for increasing the respective average speeds of horizontal and vertical ray-deflection upon variation in light intensity from elemental areas ofthe object in the sense from light to shade and decreasing such average speeds upon variation in light intensity from such areas in the sense from shade to light.

13. In a television system, an electronic device' for developing a stream ofr electrons, means for generating an electrical wave having a slope varying solely as a function of conditions of light or shade at the respective elemental area of the object scanned, and means for deecting said stream in accordance with said electrical wave.

14. In the art of television, the method of operation which comprises developing a stream of electrons and directing the same at screen structure, deecting said stream to cause the same to scan said structure, and so varying the speed of deilection in accordance with variations in conditions of light and shade from elemental areas of the object being televised that the time spent in scanning any area is directly proportional to the total light over the corresponding area of the object.

15. In a television system, a cathode ray tube including a uorescent screen and means for producing a beam of electrons rand directing them against said screen, means for simultaneously deilecting said beam horizontally at a comparatively high frequency and vertically at a comparatively low frequency whereby said screen is scanned by said beam, and means for simultaneously decreasing the speed of said horizontal deection and of said vertical deection in response to an increase in the light from an elemental area of the object being televised. f

16. In the art of television, the method of operation which comprises developing a stream of electrons and directing ther same at screen structure, deflecting said stream to cause the same to scan said structure, and varying both the instantaneous speed and the average speed of deflection in accordance withvariations in conditions of light and shade from elemental areas of the object being televised.

v ARTHUR W. VANCE. 

